audioengr
Member of the Trade: Empirical Audio
- Joined
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Quote:
Word clock is generally quite low in frequency, in the kHz. It is actually used to define the Left and Right channel multiplexing. Superclocks are generally used for the MCLK or master clock, which ranges from 11-24MHz depending on the sample rate. MCLK is usually either 128 or 256 times the word clock or L/RCLK frequency. The bit-clock is 64 times the word clock because there are 32 bits in the frame for the left and 32 bits in the frame for the right channel.
In most digital audio systems, the PLL must track the incoming data rate, whether it is from a computer or from a transport. If it doesn't, then there will eventually be underruns or overruns. The PLL has a reference clock, which may be a Superclock that sets the fundamental frequency of the PLL. However, the PLL modifies this frequency, maybe using both analog and digital techniques to match the incoming data rate. If the data is jittery, then the PLL is wandering always trying to match it. This is what creates jitter in the PLL.
The only way to completely isolate the data jitter from the clock is to buffer the data and burst the data at high-speed, or to control the speed of the source from the DAC.
Bursting at high-speed
With computers, this can be done by using USB asynchronous protocol which bursts large chunks of data at high-speed. With a transport, this can be done by reading the data at high-speed like that done on portable "shock-proof" CD players.
Controlling the source from the DAC
There are a few systems that try to do this and achieve some level of effectiveness. Lavry DAC's are one of them. The Empirical Audio Pace-Car is another. It usually requires a large buffer and control going back to the source of the data stream.
Steve N.
Empirical Audio
Manufacturer
| Originally Posted by Garbz /img/forum/go_quote.gif I do not know the differences between word and superclock, but I am more interested in this jitter thing. The PLL I know is not perfect. Jitter can leak through the PLL from the source when adjusting a VCXO to synchronise the clocks. |
Word clock is generally quite low in frequency, in the kHz. It is actually used to define the Left and Right channel multiplexing. Superclocks are generally used for the MCLK or master clock, which ranges from 11-24MHz depending on the sample rate. MCLK is usually either 128 or 256 times the word clock or L/RCLK frequency. The bit-clock is 64 times the word clock because there are 32 bits in the frame for the left and 32 bits in the frame for the right channel.
In most digital audio systems, the PLL must track the incoming data rate, whether it is from a computer or from a transport. If it doesn't, then there will eventually be underruns or overruns. The PLL has a reference clock, which may be a Superclock that sets the fundamental frequency of the PLL. However, the PLL modifies this frequency, maybe using both analog and digital techniques to match the incoming data rate. If the data is jittery, then the PLL is wandering always trying to match it. This is what creates jitter in the PLL.
The only way to completely isolate the data jitter from the clock is to buffer the data and burst the data at high-speed, or to control the speed of the source from the DAC.
Bursting at high-speed
With computers, this can be done by using USB asynchronous protocol which bursts large chunks of data at high-speed. With a transport, this can be done by reading the data at high-speed like that done on portable "shock-proof" CD players.
Controlling the source from the DAC
There are a few systems that try to do this and achieve some level of effectiveness. Lavry DAC's are one of them. The Empirical Audio Pace-Car is another. It usually requires a large buffer and control going back to the source of the data stream.
Steve N.
Empirical Audio
Manufacturer
